High energy rate forming apparatus and method



Sept- 1967 F. c. HOFFMAN ETALV 3,343,389

HIGH ENERGY RATE FORMING APPARATUS AND METHOD 3 Sheets-Sheet 1 Filed Feb. 1, 1965 INVENTORS m M M T FG F N LE O RCE C WV P V .W l. S R E E SL D R EOU R E0 E FGLM Sept. 26, 1967 F. c. HOFFMAN ETAL 3,343,339

'HIGH ENERGX RATE FORMII IG' APPARATUS AND METHOD 3 Sheets-Sheet 2 Q Filed Feb. 1, 1965 INVENTORS FREDERICK C. HOFFMAN GEORGE E. IRVING LOUIS S. MCCOLLUM MERLEfW. HESKETT Agent Sept 26, 1967 F. c; HOFFMAN ETAL HIGH ENERGY RATE FORMING APPARATUS AND METHOD Filed Feb. 1, 1965 3 Sheets-Sheet 5 FI G 4 FlG..5

N A M M w F F L somo RHVC 0. R M E E Vm S HGS D W E0 RE FG MERLE W. HESKETT 'Ageni United States Patent HIGH ENERGY RATE FORMING APPARATUS AND METHOD Frederick C. Hoffman, Los Altos, George E. Irving, Monte Sereno, Louis S. McCollum, Santa Clara, and Merle W. Heskett, Cupertino, Calif., assignors to Lockheed Aircraft Corporation, Burbank, Calif.

Filed Feb. 1, 1965, Ser. No. 429,223 8 Claims. (Cl. 72-56) ABSTRACT OF THE DISCLOSURE An improved method of high energy rate forming may be realized by arranging a workpiece to be formed in a relatively thin die member. A first explosive charge is positioned over the edge of the workpiece at a first standoff distance and a second explosive charge is positioned over the workpiece at a second standoff distance. The charges are set of such that the resulting shock wave from the first charge strikes the workpiece before the second explosive charge. The die is suspended in water by resilient supports.

The present invention relates generally to improved methods and apparatus useful in high energy rate forming. a High energy rate forming includes the use of explosives in a water medium to form a work piece into a desired shape. This technique has come into widespread usage, particularly in the aerospace industry as being a desirable method of forming parts where small production runs or relatively small quantities are involved.

Normally a die member formed from a massive block of metal is used to absorb the high-energy wave shock from an explosive charge. A work piece, for example, a flat metal sheet, is held at its edges on the draw pad portion of the die by a large clamping plate or pressure pad. This clamping plate or pressure pad is maintained at a known pressure applied to the work piece. A properly sized and shaped explosive charge is mounted over the work piece at an appropriate standotf distance. Water has been found to serve as a very useful pressure transmission medium, therefore the entire assembly is lowered to rest on the bottom of a large water tank. The explosive is detonated and the shock pressure wave resulting from the explosive force more or less pushes the work piece into the die cavity. Some of the work piece material under the pressure pad may be drawn into -the die cavity as the part was formed, depending on the pressure applied to the work piece. Excess forces from the shock pressure wave are absorbed by the massive block and the botto of the water tank.

In the preferred embodiment of the present invention, the massive and heavy die is replaced by a relatively thin shell die. This thin shell die is suspended or supported in a water medium by springs or other elastic means. The shock from the explosive force is absorbed by the water behind or under the thin die.The need for a hold-down clamp or pressure may be substituted for by the use of an elongated charge, for example, Primacord, positioned over an outer edge of the work piece to be formed. The Primaof the outer charge, being closer to the work piece, applies a force on the outer edge of the work piece to hold the work piece in position at a proper pressure. The main charge then forces the work piece into the mold of the die cavity to form the desired shape.

3,343,389 Patented Sept. 26, 1967 One object of the present invention is to provide a novel method and apparatus for high energy rate forming utilizing a relatively thin shell die.

Another object of'the present invention is to provide a unique method of holding down a work piece with an explosive force to eliminate the use of a hold-down ring or a pressure pad.

Another object of the present invention is to provide novel means for suspending the die in a water medium to absorb a shock wave thereby eliminating the need for a massive die block.

These and other objects and advantages and features of the present invention Will become apparent from a careful perusal of the fololwing specification and drawings of which: 7

FIGURE 1 is a cross-section view of a state-of-the-a'rt explosive forming die arrangement,

FIGURE 2 is a cross-section view of one embodiment of the present invention,

FIGURE 3 is a top view of FIGURE 2,

FIGURE 4 is a cross-section view of another embodiment of the present invention,

FIGURE 5 is another embodiment of the present invention showing the use of a back-up charge, and

FIGURE 6 is an embodiment of the present invention using a draw ring.

Referring to FIGURE 1, a typical high energy (explosive) forming, die arrangement is shown in cross-section. A massive, thick, bulky die member 10 of steel, steel and concrete, or similar material has a hollowed out cavity 14 formed in the top portion thereof to serve as a mold for a work piece or blank 13. Work piece 13 is securely held in place on the upper surface of die 10 by a holdd-own ring or pressure pad 11. The upper portion of die 10 supporting work piece 13 under pressure pad 11 can be referred to as the draw pad. Work piece 13 is secured in substantially airtight fashion to die 10 at the proper pressure by a plurality of spaced bolts 12 or other means which force pressure pad 11 downward. Cavity 14 is evacuated by any convenient means via line 16. An explosive charge 15 is poistioned at a proper standoff distance, D, from work piece 13 to provide the shock pressure wave which forms work piece 13 into its desired'shape. The entire apparatus is submerged to the bottom of a water tank 17' which is generally provided with a thick concrete base. A suitable electrical circuit (not shown) cord is positioned at a predetermined distance from the i is electrically connected to charge 15'to detonate the explosive charge. a

When the explosive charge is detonated, a high energy shock pressure wave forces the work piece down into cavity 14 against the solid die surface. If bolts 12 are properly torqued and pressure pad 11 exerts the proper pressure on die 13, a portion'of the work piece material under the pressure ring is also drawn into the die cavity 14 as the part is formed.

The extreme mass and bulk of die 10 and the mass of concrete base 18 absorb the excess explosive shock pressure waves thereby reducing the chance of damage to the die block.

. FIGURES 2 through 5 depict embodiments of the present invention. A relatively thin-shelled die member 20 made of steel for example, is formed into its desired shape. For example, a die8 feet across would be 2 inches thick or less for a typical operation. A work piece or blank 23 is placed on the flat upper surface or draw pad 28 of die 20 or within the cavity 24 as in FIGURE 4. A sealant material 29 is positioned around the edge of work piece 23 to provide an airtightseal between work piece 23 and die 20, after which cavity 24 is evacuated via line 26.

An explosive charge is positioned above .blank 20 at the example, a Primacord circular ring 25' is placed around the edge of work piece 23 at a standofl distance, d which is less than standoff distance D. The entire assembly is then submerged in a suitable water tank (not shown) and suspended or supported therein by a plurality of flexible support members of, for example, springs or bungees 27, which are secured to the outer edge of die 20 by eyelets 22 or other suitable means.

Explosive charges 25 and.25 are detonated at the same time by a suitable detonating device. Since charge 25', the circular Primacord charge, is closest to work piece 23, the pressure wave force therefrom strikes the outer edge of the work piece 23 first. This downward pressure force acts much the same as the force from a pressure pad. Using a suitably sized charge at a correct standoff distance, d, the pressure wave from charge 25' is regulated such that the portion of work piece 23 under the charge force will be securely held to prevent buckling. Since the standoff distance, d, is only slightly less than standoff distance, D, (for example, d=2 in., D=6 in.) the force of the center charge strikes the work piece only microseconds after the first charge. It is known that the pressure force from an explosive charge using water as the transmission medium lasts approximately 25 microseconds. Therefore, when the force of the center charge strikes the work piece, forcing it down into the cavity, a substantial force from the circular charge remains to hold down the outer edge of the work piece 23 to prevent buckling.

The shock of the explosive force from the two charges is absorbed mostly by the water which is behind or under the thin shell die. Spring member 27 allows the die to assume a substantially free floating action as the shock waves force the die member downward. It is the water which absorbs the main force of the shock, not the spring or bungee 27.

FIGURE 5 depicts an alternative embodiment of the present invention which is useful when a large explosive charge is being used to generate a shock wave of considerable energy. The basic configuration is similar to that of FIGURE 4, except a back-up charge 35 is positioned below thin shell die 20. The standoff distance from charge 35 to shell die 20 is such that both shock waves, from charge 25 and charge 35, will strike die 20 at substantially the same time. The shock wave from the back-up charge will damage die 20 when using large explosive charges.

Another embodiment of the present invention is shown in FIGURE 6. In some instances it may be desirable to maintain a great pressure on the outer edge of work piece 23. In the embodiment shown in FIGURE 6 a plurality of C clamps 32 or other means may be used to provide the desired pressure between pressure pad 31, work piece 23 and die 20. Explosive charge 25 would force work piece 23 into cavity 24 in the normal manner. If a single action stretch is desired, C clamps 32 are tightened to a high degree and no metal is drawn from under pressure pad 31 into cavity 24.

The advantages in using the apparatus and method of high-energy explosive forming as depicted in FIGURES 2-6 over the conventional or state-of-the-art configuration as seen in FIGURE 1 are many. Most obvious, of course, is a great reduction in the size and weight of the die utilized. It only follows that a thin-shelled die would "also be considerably cheaper than a massive die as shown in FIGURE 1. The advantages in elimination of the holddown ring as shown in FIGURES 2-5 are similar as a costly and awkward piece of apparatus is no longer needed in many instances.

Other advantages not so obvious are the fact that aluminum alloys of extreme hardness which do not draw easily may now be formed with high energy explosive forming methods. For example, grade T31 and grade T37 aluminum alloys have been formed into 7-foot gores which sections are made into a 7-foot diameter tank. Heretofore aluminum alloys of this alloy were formed in a softer condition and then heat-treated to acquire greater hardness.

It is contemplated with the present invention unlimited sizes of work pieces can be formed with this method. For example, it is possible to form a -foot section if a large enough body of water is available. Heretofore the great Weight of the die required Would have been impractical, even if possible. Bulging of cylindrical tubes and sizing of large diameter cylinders is possible using this thin dye concept.

In other embodiments of this invention, a series of explosive charges is placed over the work piece. The number of charges used would depend upon the size of the work piece to be formed and the hardness of the metal. Typically, it would be desirable to place the middle charge at a standoff distance greater than any of the other charges. For example, if four charges were to be used the center charge would be at the greatest distance and the outer charge at the lesser standoff distance with the distance of the other two charge rings being greater as they approach the center.

It is also contemplated that a second series of charges may be needed to eliminate any buckling at the periphery which may have occurred during the first charge, especially when using the harder aluminum alloys.

Two new forms of high energy metal forming have recently been developed which may be used with the invention concepts shown here. The first is a method of metal forming based upon direct conversion of electrical energy to mechanical energy or the so-called electrohydraulic forming process. This type of high-energy metal forming could be used as a source of high energy in place of dynamite or other explosive. The second new metal forming process is the use of high intensity transient magnetic fields. Here rapidly rising magnetic fields induce currents in the core piece. These currents create a magnetic field which is repelled by the primary field. The repulsion forces can be so great as to collapse or swage a tube and if the magnetic coil is flat a sheet metal work piece can be thrown into a die cavity.

The embodiments and examples set forth in the above specification are in no way meant to be limiting as it is believed to be well within the skill of any practitioner of the art of high energy forming to devise alternative methods utilizing the teachings contained herein.

What is claimed is:

1. A method of fabricating a metal work piece comprising the steps of: arranging a work piece to be formed Within a mold shaped in a desired figuration; arranging an explosive charge above and around the outer edge of said work piece at a first standoff distance; arranging a second explosive charge above the work piece at a second standoff distance; submerging the mold, work piece, and explosive charge in a fluid body; evacuating the region between the work piece and the top surface of the mold; and detonating said explosive charge in such a manner that the resulting explosion from the first charge strikes the work piece around the outer edge thereof before the second explosive charge strikes said work piece.

2. The method of fabricating a metal work piece according to claim 1 wherein the second standoff distance is greater than the first standoff distance and said explosive charges are detonated simultaneously.

3. A method of fabricating a metal work piece comprising the steps of: arranging a metal work piece to be formed over a mold which is shaped in a desired configuration; arranging an explosive charge above and around the outer edge of said work piece at a first standoff distance; arranging a second explosive charge above the work piece at a second standoff distance; submerging the mold, work piece and explosive charge in a fluid body; evacuating the region between the work piece and the top surface of the mold; and detonating said explosive charges in such a manner that the resulting explosion from the first charge strikes the work piece around the outer edge thereof before the second explosive charge strikes said work piece.

4. The method of fabricating a metal work piece according to claim 3 wherein the second standofi distance is greater than the first standoff distance and said explosive charges are detonated simultaneously.

5. A method of fabricating a metal work piece according to claim 3, and in addition, the step of placing a material between the outer edge of the work piece to be formed and the mold to produce a substantially airtight seal therebetween to enhance evacuation of the region between the work piece and the top surface of the mold.

6. Apparatus for the explosive forming of a metal member submerged in a body of fluid comprising: a contoured die member having a thickness which is substantially equal along its entire length, said die member being adapted to receive a metal member to be formed on the top surface thereof, means for permitting evacuation of the region between said metal member and said contoured die member, means for retaining an explosive charge above the metal member to be formed, support means adjacent said fluid body and means for suspending said die member within said body of fluid comprising a pl-urality of resilient members secured at one end to said die member and at the other end to said support means.

7. The apparatus according to claim 6 wherein said contoured die member has a thickness of & or less of the distance across said die member.

8. The apparatus according to claim 6 further including a sealant means, said sealant means positioned adjacent the outer edge of a metal work piece and the surface of said die member to aid in maintaining the evacuation of the region between said metal member and said die member.

References Cited UNITED STATES PATENTS 3,120,827 2/1964 Abegg et a1 72-56 3,136,049 6/1964 Throner et al 72-56 3,177,689 4/1965 Christian et al. 72-56 3,181,327 5/1965 Barnett et al 72-56 3,238,753 3/ 1966 Benatar et a1 72-56 FOREIGN PATENTS 1,363,060 4/1964 France.

RICHARD J. HERBST, Primary Examiner. 

1. A METHOD OF FABRICATING A METAL WORK PIECE COMPRISING THE STEPS OF: ARRANGING A WORK PIECE TO BE FORMED WITHIN A MOLD SHAPED IN A DESIRED FIGURAGION; ARRANGING AN EXPLOSIVE CHARGE ABOVE AND AROUND THE OUTER EDGE OF SAID WORK PIECE AT A FIRST STANDOFF DISTANCE; ARRANGING A SECOND EXPLOSIVE CHARGE ABOVE THE WORK PIECE AT A SECOND STANDOFF DISTANCE; SUBMERGING THE MOLD, WORK PIECE, AND EXPLOSIVE CHARGE IN A FLUID BODY; EVACUATING THE REGION BETWEEN THE WORK PIECE AND THE TOP SURFACE OF THE 